Liquid ion exchange recovery of metal values from aqueous solutions is rapidly reaching extensive commercial acceptance. Such processing has been described as being deceptively simple since all that is really happening is the transfer of a metal value from Phase A (aqueous) to Phase B (organic) and thence from Phase B to Phase C (aqueous). However, complexities of liquid ion exchange arise in a number of areas including (1) synthesis and manufacture of the reagent system, (2) evaluation of the system's capabilities, and (3) engineering application leading to large scale metal recovery.
The key to a successful application of liquid ion exchange is the reagent. In this respect, the reagent should meet a number of criteria. In the first instance, the reagent must complex with or react with a metal or group of metals. It is also desirable that the reagent show preference for a single metal where the aqueous starting solutions contain a number of metal values. The reagent should also, desirably, complex or react quantitatively with the metal under the extraction conditions. Additionally, the reagent, as well as the resulting metal complex, must exhibit satisfactory solubility in practical solvents. Further, the reagent-metal reaction must be reversible so that the metal can be stripped. For economic reasons, the reagent must be acceptably stable so that it can be recycled repeatedly. Also, it should be essentially water insoluble to prevent significant loss into the aqueous phase. Furthermore, the reagent should not cause or stabilize emulsions. And, of course, the cost of the reagent should be such that the liquid ion exchange process can be operated at a profit. Very few compounds have, as yet, found significant commercial acceptance.
Certain benzoxazoles are known. These are generally prepared by condensation of a salicylic acid compound, such as, salicylamide with an aminophenol. This is generally described in The Journal of Physical Chemistry, Vol. 74, No. 26, (1970), pp. 4473-4480. In U.S. Pat. No. 2,995,540, there are disclosed certain benzoxazoles which are useful for protection against ultra-violet radiation. These benzoxazoles contain lower alkyl groups such as methyl groups. In Analytica Chemica Acta, Vol. 11, (1954), pp. 301-308, there is reported a study of the stability of chelates of 2-(o-hydroxyphenyl)-benzimidazole and analogous reagents, one of such analogous reagents being 2-(o-hydroxyphenyl)-benzoxazole.